1. Field of the Invention
The present invention relates to a system and method for accurately identifying the location of a mobile wireless device in a network, in particular, an ad-hoc wireless mobile communication network, using received signal strength indicator and signal propagation time. More particularly, the present invention relates to the employment of such a system and method in a real-world application, such as a fire and rescue scenario, to quickly and accurately identify the floor number where a firefighter in need of help is located.
2. Description of the Related Art
Wireless communication networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base node to communicate simultaneously with several mobile nodes in its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc multi-hopping” network has been developed. In this type of network, each mobile node is capable of operating as a router for the other mobile nodes providing most of the functionality of a base station, thus expanding the coverage area with very little cost. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other fixed or mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc multi-hopping networks are described in U.S. patent application Ser. No. 09/897,790 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, filed on Jun. 29, 2001, in U.S. patent application Ser. No. 09/815,157 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, filed on Mar. 22, 2001, and in U.S. patent application Ser. No. 09/815,164 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, filed on Mar. 22, 2001, the entire content of each application being incorporated herein by reference.
Either in conventional wireless communications networks, or in ad-hoc wireless communications networks, it may be necessary or desirable for a mobile node to be capable of knowing or determining a relative or absolute geographic location or position. As known to those skilled in the art, this can be achieved through the use of a number of technologies. These technologies require cell identification, combined with Round Trip Time (RTT), Timing Advance (TA) and Measured Signal level (RX level), Time Difference of Arrival (TDOA) and Angle Of Arrival (AOA) techniques, the details of which can be appreciated by one skilled in the art. Another available technology uses cellular signal timing based methods for code division multiple access (CDMA) and wideband code division multiple access (WCDMA). Yet another technology uses Global Positioning System (GPS) techniques, which is generally viewed as being more accurate than all other methods listed.
Despite the fact that the GPS technique has been in use for a considerable period of time and most of the world's navigation relies on it, the GPS technique is very susceptible to large errors in measurement in some specific conditions. It can provide location determination results with very high accuracy only after performing a relatively large number of measurements involving a large number of satellites to remove propagation and method errors. A description of the shortcomings of GPS is set forth in a document by the Institute For Mathematics and its Applications (IMA) entitled “Mathematical Challenges in Global Positioning Systems (GPS)”, the entire content of which being incorporated herein by reference. Certain other tests also demonstrate that the GPS technique is unsuitable for those terrestrial-based networks operating in an environment where the number of visible satellites is too small for providing good precision as in underground tunnels, inside buildings, under heavy foliage or in urban “canyons”.
To overcome the above issues with determining location information, new techniques are being developed which do not require either the use of satellites or a centralized computing facility for determining location information. Further details of new techniques for computing location of a mobile terminal in ad-hoc multi-hopping networks are described in U.S. Pat. No. 6,728,545 entitled “System and Method for Computing the Location of a Mobile Terminal in a Wireless Communications Network”, the entire contents of which is incorporated herein by reference. Additionally, ad-hoc networks can be developed utilizing non-fixed, or movable infrastructure components. Further details of networks using movable access points and repeaters for optimized coverage and capacity constraints are described in U.S. patent application Ser. No. 09/929,030 entitled “Movable Access Points and Repeaters for Minimizing Coverage and Capacity Constraints in a Wireless Communications Network and a Method for Using the Same”, filed Aug. 15, 2001, the entire content being incorporated herein by reference.
The patent and patent applications discussed above generally relate to mobile networks that connect to a permanent fixed network where location information is presented as absolute locations. However, as can be appreciated from the patent applications referenced above, temporary ad-hoc multi-hopping networks do not necessarily have the same requirements. Therefore, a need exists for a portable, easily deployed, self-contained ad-hoc multi-hopping network system where relative location detection is desired, such as where the location of personnel operating in emergency condition is critical. The relative location may be provided in addition to, or in replacement of absolute geo-location, and should be readily communicable between and among various transmission obstacles typically present in such location.
Accordingly, a need exists for an improved system and method for easily determining and communicating the absolute and/or relative location of a mobile node in a deployed wireless communications network.